Microwave attenuator



Patented Jan. 12, 1954 BHCROWAVE ATTENUATOR Stephen G. Sydoriak, New Haven, Conn., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application May 8, 1946, Serial No. 668,149

8 Claims. (01. 333-81) This invention relates generally to a radio frequency attenuator and, more particularly, to a reflection type attenuator having an iris coupling.

One type of radio frequency attenuator employs two loops of variable separation in a wave guide having a critical size (below cut-off frequency). One loop excites an electromagnetic field within the guide, and the second loop absorbs energy by an amount related to the separation of the two loops. The electrical field excited by the input loop decays in an exponential fashion at positions away from the loop. The energy absorbed by the second loop is approximately proportional to the square of the electric field at the loop position. A control of the attenuation is thus achieved by adjusting the relative positions of the two loops.

The electrical energy excited by such a loop is primarily of the TE1,1 mode, although various undesirable modes such as the TM0,1 mode are also present. The presence of these undesirable modes produces appreciable departure from linearity of the device. One object of the present invention is to provide a reliable attenuator which is excited in the TE1,1 mode by means of an iris opening, and whose attenuating properties are controlled by means of a calibrated sliding member.

Certain types of radio frequency attenuators are characterized by having a high input insertion loss, or insertion attenuation, resulting to some extent in the presence of the undesirable modes. Another object of the present invention is to provide a radio frequency attenuator which decreases the efiect of the undesirable modes to a negligible value, and thereby substantially reduces the insertion loss of the device.

For an attenuating device having sliding or moving members to vary the magnitude of attenuation, the end effect characteristic of the member becomes appreciable for values of attenuation close to zero. End eifect result from the change of the configuration of the electrical field in an enclosure as the geometry of the enclosure is changed. It is another object of the present invention to reduce the undesirable end effect characteristics inherent in such a device.

Various effects due to poor mechanical contacts, leakage, and standing waves inherent in an attenuating device should be eliminated or compensated for, since they tend to make the value of input power level vary and indeterminate. Still another object is to provide a radio frequency attenuator which allows or compensate for the described undesirable characteristics.

An attenuator is sometimes used between adjacent circuit elements. The input impedances of said elements often differ widely, and result in a high degree of mismatch between the elements. Such mismatch produces a varying degree of coupling as a function of frequency, and it is therefore undesirable. A lossy line will reduce this uncertainty in power level, and may be used for such a purpose. A lossy line may be any line comprising a resistive material and offering a predetermined degree of attenuation to a radio frequency signal applied thereto. Still another object is to provide an attenuator which will eliminate the need or obviate the use of a section of such a lossy line.

Other objects, featurs and advantages of this invention will suggest themselves to those skilled in the art and will become apparent from the following description of the invention taken in connection with the accompanying drawing, in which:

Fig. 1 shows a cross-sectional View of one em bodiment of a radio frequency attenuator em bodying principles of the invention;

Fig. 2 shows a cross-sectional view of a second embodiment of a radio frequency attenuator embodying principles of this invention;

Fig. 3, comprising Figs. 3A and 3B, shows another embodiment of a radio frequency attenuator embodying principles of this invention; and

Fig. i shows a fragmentary, enlarged crosssectional view, taken along line A-A of Fig. (4.

Referring more specifically to Fig. 1, there is shown a coaxial line Iii having an outer con" ductor H and an inner conductor i2. Gne end of the coaxial line is short circuited by means of a metallic disc [3 which is joined to the center conductor and the outer conductor in a plane transverse to the axis of the line. At the other end of the line is a suitable coupling means ii for joining the attenuator to an adjacent circuit element. Coupling means it may include a screw tube I! in the outer conductor ii.

connector ii) on outer conductor ii, and a slip friction joint it on center conductor it.

At a point approximately one-half wavelength from the inner surface of disc l3, a hollow metallic attenuator tube I1 is joined at right angles to outer conductor ii. A small slot is perpendicular to the axis of outer conductor ii is located at the geometric center of the area inscribed by Slot id constitutes an iris coupling means between coaxial line In and the inside of tube ll.

Part of variable slide attenuating mechanism 20 is internal to tube I7, and its remainder extends to the outside of tube ll. The outer end of mechanism 2d includes a coupler 2%, which may be similar to coupler M. Coupler 2| may include a screw base 22 on a section of outer conductor 24, and slip joint 23 on a section of center conductor 25. To outer conductor 24 is at tached a hollow tube "6 with a flared section 29 on the end of the tube remote from outer con.- ductor 24. The flared end is in mechanical contact wtih the inside surface of tube H, and is free to slide along the inside of tube ll. Center conductor 25 is extended in the form of a conductor El axially and centrally of tube 28. In the vicinity of the flared end 29- of' tube 25, conductor 2t is bent to form a loop- 28,. and'the free end is attached to the end of the flared section.

The iris l3 inhibits excitation of all modes except the TEi,1 mode so that wide variation in slider loop design is possible so far as linearity of attenuation is concerned. The calibrator is independent of the direction of propagation through the device, for, if the loop 28 is on the input end, it will set up the undesirable TMo,1 mode, but the iris will not pass this mode, and no energy will be transferred. The standing wave ratio (S. W. R.) looking into the device shown in Fig. l is high, and therefore sumcient lossy lines must be placed on both ends of the device. The value of attenuation may be adjusted by means of a calibrated knob '59 which may be connected to mechanism 2% by a suitable ratchet-drive as sembly, not shown.

Referring more specifically to Fig. 2 there is shown a second embodiment of the invention, in which a coaxial line 36 has an outer conductor 3i and a center conductor 32. One end of the line is short circuited by means of a metallic disc 33 which is joined to outer conductor 3! in a plane transverse to the axis of the line. The other end of the line is of tapering or flared-type construction which forms a tapering or flared section 3'! of coaxial line adjacent to this end-of the line. The type of construction described affords a flared section 31 and an enlarged section 38" at opposite ends of the line. At the flared end of the line, there is a suitable coupling means M for joining the attenuator to an adjacent circuit element. Coupling means 34 may be similar to coupler l lshown in Fig. l and include a screw connector 35 on outer conductor 31 and a slip joint 36 on center conductor 32.

In the enlarged section 38, a section of tubular attenuator conductor 39 is inserted through a hole in outer conductor 3!. One end of tube 39 projects into the coaxial line in the region between outer conductor 3| and center conductor 32. The tube 39 is terminated by a thin disc 40 having a small slot or iris il, the parallel sides of which are perpendicular to the axis of coaxial line 30.

Part of a sliding attenuating member 42 is internal said tube 39, and its remainder extends to the outside. Member 42 shown in Fig. 2 may be similar to member 28 shown in Fig. 1, and those similar parts bear similar reference numerals.

In the attenuator, the iris is formed by effectively cutting through the outer conductor of the enlarged section 38. The magnetic field in the vicinity of the iris has a high curvature if section 38 is not large and there is therefore an associated end effect on this account. The end effect is reduced by swelling out the enlarged section of the coaxial line in the manner shown in the figure so that a planar iris can be used. By adjustment of the iris towards the center conductor, the insertion loss can be reduced.

Fig. fl is a fragmentary cross-sectional view taken along line AA of Fig. 2 and showing the iris with the aperture therein. The outer conductor 3| is shown in cross-section, while disc if! is shown communicating therewith and as having a slot 4] therein. Slot 4| will pass only energy having the desired TE'1,1 mode.

The embodiment of the attenuator entailing the principles shown in Figs. 3A and 3B possesses the desirable characteristics of the embodiments shown in Figs. 1 and 2 in addition to several others. Referring to Figs. 3A and 33, a completely enclosed cylindrical cavity 49 is formed by cylindermember 50' onto whose ends are attached planar end members or walls 5i and 52. The top View of cavity 49 is shown in Fig. 3A. Leakproof tuning mechanismit is inserted through a hole in member 52 so that its end projects into the interior of cavity 49. Tuning mechanism includes a metallic outer housing 5 of bullet type shape which is affixed to and in mechanical contact with member 52.

A non-metallic shaft 55 extends through a small hole or passage in the center of housing 54 and projects a short distance within the cavity 49. At the inside end of shaft 56 is attached a metallic slug 55 whose position within the cavity is adjustable by means of shaft 56. The position of shaft 55 may be adjusted from outside the cavity by means of screw 51 attached to the outer end of shaft 55. The outer conductor 58 of coaxial line 6Q is joined at right angles to cylinder 5 9. A loop of conductor 62 is attached to the end of center conductor t! of line 60' and to outer conductor 58, the loop projecting within the cavity; Coupler 59 may be similar to coupler 14 shown in Fig. 1. Diametrically opposite loop 62 attenuator mechanism 63' projects through a hole or opening in cylinder 50.

Slider assembly 63 includes a metallic tube 3 3 which projects a short distance within the cavity 49 and is in mechanical contact with cylinder 50. The end of tube 65 is terminated by. a thin flat plate 65. Plate 65 includes a slot 99, whose parallel sides are perpendicular to the axis of cylinder, 50'. This structure forms a coupling means for electromagnetic energy between the cavity and the inside of tube 64. The slider assembly includes a hollow metallic tube 6% of smaller dimensions than tube 64. The outer end of tube 65. includes a coupling means H which may be similar to coupler [4 described in connection with Fig. l. The inside end. of tube 55 is terminated by a disc 69, which contains a resistive coating or film 12 on the planar surface nearest the cavity 49. concentrically spaced center con.- ductor 6? of tube 66 projects a short distance through a hole or aperture in disc 69'. The inner end of conductor 61 is terminated by means. of a loop of conductor 68. 'Tube 66 is held inpl'ace-and guided inside tube 64 by means of two annular, coil springs oriented at right angles to the axis of the tubes. Springs 10' are in mechanical contact with the outer surface of tube 56 and the inner surface of tube 64, and allow tube 66 to be adjusted relative to tube 64.

The initial insertion loss of an iris attenuator is reducedconsiderably by coupling the iris through a high resonant cavity. The cavity has the function of building up the electrical potential across'the edges. of the iris, to a very high value. The iris-ended attenuator tube projects into the cavity. If the iris has the proper width and extends into the cavity the proper amount, a. very low insertion loss is obtained.

A fixed tuned cavity may be used although it has been found preferable in the particular emmodiment to use the tunable cavity shown.

Detuning the cavity does not change the linearity characteristics, but it does change the inserticn loss. A. cavity having a low Q for tuning purposes is desirable, and this may be accomplished by enlarging the cavity loop 62. Tuning over a very broad range of frequencies is accomplished by adjusting the position of slug 55.

Leakage is reduced considerably by completely enclosing the cavity by the metallic Walls, by the leak-proof tuning mechanism shown. The long insulated shaft 56 of the tuning mechanism which enters the cavity through a small diameter hole provides a dielectric filled, below cut-off frequency wave guide which suppresses or minimizes leakage of energy through such wave guide. The slider assembly shown in Figs. 3A and 3B has been found to reduce the leakage through the slider member considerably. The standing wave ratio looking into the slider is reduced considerably by the introduction of the resistive disc. With the standing wave ratio reduced, the amount of lossy line required to prevent interaction with other circuit elements in which the attenuator is used in greatly reduced.

The attenuator may incorporate a monitor attachment. The current induced in an additional loop til coupled into the cavity is rectified by a crystal 8! attached to it, and measured by a microammeter 82. For a given cavity geometry, the monitor is a measure of the electrical potential across the iris.

The value of attenuation may be indicated by a knob dial 19 attached to the slider. It has been determined by experimentation that the TE1,1 mode is predominant, the linearity characteristics are good, and that the angle calibration of the dial may b quite accurate.

Among the factors which determine the departure of the device from linearity and, hence, the insertion attenuation of the attenuator, are undesirable modes, transformer action, and loading. The introduction of the iris coupling elimihates substantially all modes but the TE1,1 mode. The insertion attenuation is reduced by using a high Q cavity to build up the potential across the iris. The attenuator reduces the apparent loading characteristics by the proper adjustment of the geometric features Within the cavity. The end effect at small antenna separations is reduced by having the iris contained in one plane; inserting the iris assembly to the correct depth as determined by a comprise between good linearity characteristics and low insertion losses; having the appropriate width of the iris determined by a similar comprise; and having the orientation of the cavity loop substantially parallel to the axis of the cavity, as determined by a compromise between maximum coupling and the optimum Q for practical tuning. Optimum Q is obtained by a compromise between a high Q for maximum transformer action and a low Q for practical tuning purposes. Irregularities in the cavity walls and the addition of the resistive disc do not appreciably change the end effects. The introduction of the lumped resistance in the form of a resistive disc reduces the standing wave ratio considerably and makes it possible to remove some of the lossy cable between the slider and adjacent circuit elements.

While there has been described hereinabove what is at present considered to be a preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention.

What is claimed is:

1. A radio frequency attenuator comprising a section of coaxial transmission line having an outer conductor and an inner conductor, an attenuating tube, said tube being joined to said outer conductor, an iris coupling means between said line and the end of said tube joined to said line, said iris coupling means including an apertured disc for only passing energy in the TE1,1 mode, a slider tube having a flared section at the inside end and adapted to slide inside said attenuating tube, a center conductor concentrically spaced inside said slider tube, said conductor being terminated at its inside end by a loop, means for adjusting the position of said slider tube with respect to said attenuator tube, coupling means at the outer end of said slider tube, and coupling means at one end of said line.

2. The attenuator of claim 1 wherein said coaxial line includes a tapering section.

3. A radio frequency attenuator comprising a first section of coaxial transmission line, a flared section of coaxial transmission line, said flared section being joined to said first section at one end and including a coupling means at the other end, an attenuator tube, said tube being joined to said first section and projecting therein at its inner end, the inner end of said tube being terminated by a disc having an iris aperture therein for passing only energy in the TE1,1 mode, a slider tube having a flared section at its inner end and a coupling means at the other end, said slider tube flared section being adapted to slide inside said attenuator tube, a center conductor concentrically spaced inside said slider tube, said conductor being terminated at its inner end by a loop, means for adjusting the position of said slider tube with respect to said attenuator tube, and coupling means at the outer end of said slider tube.

4. A radio frequency attenuator comprising a cylindrically shaped cavity resonator, an adjustable tuner projecting into said cavity resonator, a coupling loop projecting into said cavity resonator and adapted to transfer electromagnetic energ an attenuator tube, said tube being joined at its inner end to a wall of said cavity resonator and projecting into said cavity, the inner end of said tube being terminated by an iris coupling means including a slot elongated in a direction perpendicular to the axis of said cylindrically shaped cavity resonator for passing only energy in the 'IE1,i mode, a slider tube adapted to slide inside said attenuator tube, said slider tube having a coupling means at its outer end and a load disc having a resistive deposit thereon at the other end, a center conductor concentrically spaced inside said slider tube, the inner end of said conductor projecting through said load disc and terminated by a loop, and means for adjusting the position of said slider tube with respect to said attenuator tube.

5. A radio frequency attenuator comprising a section of coaxial transmission line having an outer conductor and an inner conductor, said coaxial line including a tapering section, an attenuating tube, said tube projecting within said coaxial line and being joined to said outer conductor, a disc terminating said attenuating tube within said coaxial line, said disc having a slot therein for coupling said line and the end of said tube joined to said line, a slider tube having a flared section at its inside end and adapted to acetate;

slide inside said attenuating tube, a center conductor concentrically spaced inside. said: slider tube, said conductor being terminated at. its inside end by a loop, means for adjusting-the position to said slider tube with respect to said attemuator tube, coupling means at: the outer end of said slider tube, and coupling means. at one endoi said'line.

6'. A radio frequency attenuator comprising; a cylindrical chamber, anattenu-ating tube, one; end. of said tube being connected to. said chamber; an iris coupling means between. said chamber and. said one end of said tube, said. coupling means including slotenlongated in a: direction. perpendicular to. the axis oisaidcylind-rical chamher, a. slider tube. adapted. to slide: inside; said attenuating tube, a center. conductor concentrically spaced inside said slider tube, said conductor being terminated at its inner end: by aloop,v and.

means for adjusting the position of said.- slider- 20 tube with respect to said attenuator tube.

7 The attenuator of claim 6,. wherein said' cylindrical chamber has a substantially uniform cross-sectional area. along. its length.

8. The attenuator of claim 6 further including:

100p coupling means coupled to said chamber and: means: for tuning said chamber to. a. desired frequency.

STEPHEN G.. SYDORIAK.

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